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NTSB: Airplane radar not always accurate

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Such outdated weather information may have played a role in two recent fatal crashes

The problem does not impact large commercial jetliners

Weather imagery that private pilots see on cockpit displays -- advertised as "real time" data by some firms -- could be up to 20 minutes old, giving pilots a dangerously false sense of existing conditions, federal safety officials warned this week.

While pilots generally know that weather information is five minutes old, the actual age can be much older, the National Transportation Safety Board said in a safety alert. That "could mean the difference between life and death" in rapidly changing weather conditions, said NTSB Chairwoman Deborah A.P. Hersman.

The problem affects thousands of general aviation aircraft, ranging from small two-seat aircraft to large corporate jets, which use Next Generation Radar, or NEXRAD, systems. It does not impact large commercial jetliners, which use other weather-monitoring systems.

Old weather information may have played a role in at least two fatal crashes in recent years, the NTSB said.

In March of 2010, a med-evac helicopter crashed in Tennessee when the pilot tried to beat a storm back to his home base. And in December of 2011, a Piper PA-32 suffered an in-flight breakup in stormy weather near Bryan, Texas.

In each case, the NTSB said, the pilots may have misinterpreted a time stamp on the weather monitor. The time stamps indicated the one-minute time interval used to create the image, and not the actual age of the data used to create the image.

The pilots may not have realized that the images contained much older data, the NTSB said. In the helicopter crash, the weather data available to the pilot was about five minutes old. In the Piper crash, the data was between six and eight minutes old.

In the case of the Piper, the weather display would have indicated that the line of rain showers was one mile east of the aircraft at the moment the Piper crashed in a rain storm.

Both crashes occurred at night, when darkness would have obscured the storms.

"We believe this is really a pitfall for pilots, that they're getting information that actually has a stamp on it that's telling them 'It's one minute old.' 'It's five minutes old.' But in reality the information used to create the image could be significantly older. We think it's important for them to know that," Hersman said.

Aviation experts contacted by CNN said the NTSB alert likely will have little impact on cautious aviators who give wide berth to dangerous thunderstorms, but could impact riskier fliers.

"The conventional wisdom is any severe weather you want to avoid by about 20 miles," said Bruce Landsberg, president of the AOPA Foundation and Air Safety Institute, the nonprofit education arm of the Aircraft Owners and Pilots Association.

Even given a 20-minute lag in weather information, the 20 miles will provide an adequate cushion, he said.

But "if you are prone to cutting corners, particularly around severe weather, (a misunderstanding of the delay in weather data) could be dangerous," Landsberg said.

"There is some validity" to the NTSB's concerns, he said. "I wouldn't argue the point. Obviously our best defense against people having accidents is a good education," Landsberg said.

"I think we have an opportunity here to take a look at this and say, 'OK, what does this mean' and put this in terms that the average pilot would understand," he said.

Rob Wefelmeyer, a flight instructor at Maryland's Freeway Airport, said pilots need to be aware that Next Generation Radar (NEXRAD) weather data is old. "You don't want to use it to navigate around weather. They have to be very aware that that is old info," he said.

Hersman said modern weather-monitoring systems have increased safety.

"We think this is great technology. We think it can be very helpful to pilots," she said. "They just need to be sure that they have all the correct assumptions about how that information is displayed to them and what it's based on."